In the semiconductor industry, different wet chemical etching methods and/or dry etching methods are used for surface treatment, especially surface cleaning and surface activation. Most notably, these assist in the removal of native surface oxides and/or carbon compounds. The significant difference between the wet chemical etching methods and the dry etching methods is the physical state of the cleaning agents used. In the case of a wet chemical etching method, liquids are used to clean the substrate surface; in a dry etching method, gases or plasmas are used to do so. Wet chemical etching methods are still justified in a few cases; in recent years, however, they have been replaced by dry etching systems in many other fields.
For dry etching, primarily plasma systems or ion sputtering systems are used.
A “plasma system” is understood as a system that can generate a quasi-neutral gas—a so-called plasma. By applying a voltage between the two electrodes, the process gas is ionized and forms an ionized, quasi-neutral gas—the so-called plasma. These systems are referred to as “capacitively coupled plasma” (CCP) plasma systems. Generating plasma by means of magnetic devices in so-called “inductively coupled plasma” (ICP) plasma systems is also conceivable. In this case electromagnetic induction is used to manufacture a plasma. Furthermore, still other methods for generating plasma exist; these methods will not be elaborated upon here. The statements made hereinafter generally apply to all plasma systems, but will, however, be described with the help of CCP plasma systems, as the construction of such is relatively simple.
By means of its distinctive characteristics, this plasma acts on the surface of a substrate that is preferably located on or in the vicinity of one of the two electrodes. In general, either inert or reactive process gases can be used. In the case of a plasma system, the voltage between two electrodes, the gas composition, and the gas pressure are preset. Low-temperature plasmas thereby ignite above a critical pressure. This critical pressure is large in comparison to the pressures in a high vacuum chamber, but less than 1 bar. Low-temperature plasmas are mostly composed of oxygen, nitrogen, inert gases, or more complex organic gaseous compounds. These modify the substrate surface both physically, through ion bombardment, and chemically, through radicals generated in the plasma (source: S. Vallon, A. Hofrichter et. Al, Journal of Adhesion Science and Technology, (1996) vol. 10, no. 12, 1287). The physical modification is, above all, to be ascribed to the high speed and accompanying impact energies between gas- and plasma atoms and the atoms of the substrate surface.
Ion sputtering systems, on the other hand, have an ion source and an acceleration unit. In the ion source, a process gas is ionized and accelerated via the acceleration unit in the direction of the substrate surface. The accelerated ions form the so-called ion beam, which has a mean diameter, a corresponding divergence or convergence, and an energy density. The accelerated ions can loosen impurities from the substrate surface when the kinetic energy of the ions is greater than or at least as great as the bond energy between the impurity and the substrate surface. In this process, the kinetic energy of the ions can, however, also affect the substrate surface itself. This manipulation manifests itself in the alteration of the microstructure of the substrate surface, in the generation of point defects, incorporated ions in the crystal lattice, plastic deformation, etc.
The removal of impurities is called sputtering. For the sake of completeness, it should be noted that the separation process of atoms on surfaces can also be termed sputtering. Hereinafter, however, sputtering will only refer to the removal of atoms.
One technical problem is that, in the given environmental conditions, a reaction between the impurities (that are to be removed) on the substrate and the ions can occur without the impurities being removed from the substrate surface. Furthermore, the ions usually also react with the base material that is to be cleaned, and that is located underneath the impurities that are to be removed. This results in an inhomogeneous removal, and thus an increase in surface roughness.
Furthermore, adsorption of the gas and plasma atoms on the surface of the substrate often occurs.
According to the state of the art, the removal of impurities, especially by means of hydrogen, is done primarily using the following methods:
Plasma Processes
    1. The substrate surface is irradiated by means of a hydrogen plasma. The temperature of the surface is thereby usually very greatly increased by the plasma. In general, different plasma processes are used for different materials. For InP, for example, a so-called ECR (electron cyclotron resonance) plasma (source: A. J. Nelson, S. Frigo, D. Mancini, and R. Rosenberg, J. Appl. Phys. 70, 5619 (1991)) is used. The removal of all surface impurities from GaAs was, after hydrogen irradiation by means of an RF (radio-frequency) plasma, observed at a temperature of 380° C. after approximately 30 minutes (source: S. W. Robey and K. Sinniah, J. Appl. Phys. 88, 2994 (2000)). For CuInSe2, the removal of surface oxides is described, for example, through the use of a hydrogen ECR plasma at a sample temperature of 200° C. (source: A. J. Nelson, S. P. Frigo, and R. Rosenberg, J. Appl. Phys. 73, 8561 (1993)).    2. Under the action of molecular hydrogen, the substrate surface is heated in a vacuum to more than 500° C. With GaAs, for example, a cleaning time of up to two hours is necessary (source: DE 100 48 374 A1).    3. The substrate surface is heated using atomic hydrogen in a vacuum. With GaAs, this takes place, for example, preferably in a temperature range between 350° C. and 400° C. This method is extensively detailed in the references (sources: Y. Ide and M. Yamada, J. Vac. Sci. Technol. A 12, 1858 (1994) or T. Akatsu, A. Plöβl, H. Stenzel, and U. Gösele, J. Appl. Phys. 86, 7146 (1999) and DE 100 48 374 A1).Ion Beam Processes
The removal of surface impurities through the use of inert gases (Ar, N2, xenon, . . . ) of an ion beam process is predominantly grounded upon the basis of a pure sputter removal (source: J. G. C. Labanda, S. A. Barnett, and L. Hultman “Sputter cleaning and smoothening of GaAs(001) using glancing-angle ion bombardment”, Appl. Phys. Lett. 66, 3114 (1995)). The use of reactive gases (H2, O2, N2, CF5) is, on the other hand, predominantly based on a chemical reaction of the process gas with the surface impurities and the subsequent removal of the reaction products (desorption, stimulated partially thermally or by ion bombardment). Hydrogen has turned out to be a preferred process gas for the removal of various surface oxides and carbon impurities on the surface of semiconductors (source: DE 10 210 253 A1).
The named state-of-the-art methods of surface-cleaning serve, in general, to prepare the substrate surface for a subsequent process. Subsequent processes are, for example, a coating with a photoresist, a deposition of one or more atoms or molecules by means of a vapor deposition process such as PVD (physical vapor deposition) or CVD (chemical vapor deposition).
Due to the ions, surface defects are generated above all in semiconductor materials, which defects can critically impair the electric function of components. Further, in the current state of the art, the fundamental problem arises that plasma systems and sputter systems always modify the substrate surface fundamentally, and usually damage it.
When plasma process or ion sputtering processes are used to clean or activate Si—, SiC—, Quartz-, or 3A-5B-semiconductor surfaces, for example, additional damage occurs as a substantial disadvantage on the surface of the semiconductor (e.g. roughness, formation of metallic phases, oxide layer growth, formation of a thin water film). Admittedly, verifiable, efficient surface cleaning of most impurities does take place through the use of the plasma systems; however, it becomes apparent, especially with organic components, that this cleaning method is not sufficient to completely remove the organic connections.
The primary reason for the generation of damage when using plasma systems is essentially the direct contact between the plasma and the semiconductor surface and the resulting interaction between plasma components (for example, ions, radical and highly excited molecules/atoms, electrons, and UV photons) and the atoms on the substrate surface. In particular, the highly excited and therefore highly energetic particles or the ions of a plasma or the particles or ions of an ion sputtering system generate, with high probability, disadvantageous damage processes of the substrate surface, especially a highly sensitive semiconductor surface.
The low-temperature plasmas, generated using oxygen, nitrogen, inert gases, or more complex organic gaseous compounds, modify the substrate surface through ion bombardment as well as through surface reactions using radicals that exist in the plasma (source: K. Harth, Hibst, H., Surface and Coatings technology (1993) vol. 59, no. 1-3, 350). Additionally, gas atoms on the surface are absorbed (source: K. Scheerschmidt, D. Conrad, A. Belov, H. Stenzel, “UHV-Silicon Wafer Bonding at Room temperature: Molecular Dynamics and Experiment”, in Proc. 4. Int. Symp. on Semiconductor Wafer Bonding, September 1997, Paris, France, p. 381).
The crucial disadvantage of the methods for cleaning semiconductor surfaces under the influence of atomic or molecular hydrogen in a vacuum is the high temperatures that are necessary and that usually lie between 400° C. and 500° C. (see: DE 100 48 374 A1). The high process temperatures are necessary to enable both an effective reaction between the hydrogen and the impurities and the desorption of parts of the resulting reaction products.
A further disadvantage of the described state-of-the-art methods for cleaning semiconductor surfaces using molecular and/or atomic hydrogen is the long process times; in the case of molecular hydrogen, these amount to at least two hours (see: DE 100 48 374 A1).
The use of the embodiment according to the invention from the patent specification DE 102 10 253 A1 is not efficient enough, as removing the impurities through a purely chemical process by means of hydrogen ions of Si, SiO2, SiC quartz, and Zerodur does not lead to complete removal.